In recent years attention has been paid to the environmental contamination and the high energy consumption especially due to the dramatically occuring forest death. Exhaust gases have however always been a problem in population centers. Despite continuously improved engines and heating techniques with less discharges or exhaust gases, the increasing numbers of vehicles and incineration plants have resulted in a total increase of the amount of exhaust gases.
The primary cause of impure exhaust gases and high energy consumption is insufficent combustion. The construction of the combustion process, the efficiency of the ignition system, the quality of the fuel and the fuel-air mixture determines how effective the combustion becomes and how much uncombusted and dangerous substances the gases contain. Different techniques are used for reducing the amount of the substances, for examples recirculation systems and the well known catalyst technique, which provides a combustion of the exhaust gases outside the actual combustion process.
Combustion is the reaction of a substance with oxygen (O.sub.2) with generation of heat. Substances like carbon (C), hydrogen gas (H.sub.2), hydrocarbons and sulphur (S) generate sufficent heat for maintaining their combustion temperature while for example nitrogen gas (N.sub.2) requires a supply of heat for being oxidized.
At a high temperature, 1200.degree.-2500.degree. C. and a sufficient amount of oxygen a complete combustion is achieved, i.e. each substance takes up a maximum amount of oxygen. The end products are CO.sub.2 (carbon dioxide), H.sub.2 O (water), SO.sub.2 and SO.sub.3 (sulphur oxides) and somewhat NO and NO.sub.2 (nitrogen oxides, NOx). The sulphur and nitrogen oxides are responsible for a great deal of the acidification of the environment; they are injurious to inhale and especially the later steals energy from the combustion process.
It is also possible to obtain cold flames, for example the blue, flattering flame from a candle which is going out, where the temperature is only about 400.degree. C. The oxidation will then not be complete but the end products can be H.sub.2 O.sub.2 (hydrogen peroxide), CO (carbon monoxide) and possibly C (soot). The two last mentioned substances are, like NO, injurious and can give off more energy at a complete combustion.
Petrol is a mixture of hydrocarbons from crude oil with a boiling point in the interval 40.degree.-200.degree. C. About 2000 different hydrocarbons with 4-9 carbons are contained therein.
The detailed combustion process is very complicated also for simple substances. The fuel molecules are decomposed to smaller units, of which most are so called free radicals, i.e. unstable molecules which quickly react with for example oxygen.
The most important radicals are the oxygen atom O: the hydrogen atom H and the hydroxyl radical OH. Especially the latter is of greatest importance for decomposing and oxidizing the fuel both through directly bonding to this and by removing hydrogen at which water is formed.
In the beginning of the initiation of the combustion water is decomposed from these according to the reaction EQU H.sub.2 O+M.fwdarw.H.+OH.+M
where M is another molecule for example nitrogen or a wall or a spark plug electrode with surface which the water collides. Since water is a very stable molecule a high temperature is required for the decomposition to take place. The better alternative is to add hydrogen peroxide which is decomposed in a similar way: EQU H.sub.2 O.sub.2 +M.fwdarw.2OH.+M
This reaction takes place much easier and at a lower temperature, especially on surfaces at which the ignition of the fuel-air mixture occurs easier and in a more controlled way. A further positive effect of the reaction of a surface is that hydrogen peroxide easily reacts with soot and tar on walls and spark plugs to carbon dioxide (CO.sub.2) which gives clean electrode surfaces and a better spark.
If both hydrogen peroxide and water is added a drastic reduction of CO in the exhaust gases is provided according to the following: EQU CO+O.sub.2 .fwdarw.CO.sub.2+ O: initiation 1) EQU O:+H.sub.2 O.fwdarw.2OH. branching 2) EQU OH.+CO.fwdarw.CO.sub.2 +H. propagation 3) EQU H.+O.sub.2 .fwdarw.OH.+O: branching 4)
From the reaction 2) we can see that water plays a catalyzing role by later being reformed. Due to that hydrogen peroxide gives a many thousand times higher content of OH-radicals than water step 3) will be considerably accelerated and remove the most of CO formed. By that additional energy is set free which helps to maintain the combustion.
NO and NO.sub.2 are very poisonous compounds, about four times as poisonous as carbon monoxide. At an acute poisoning the lung tissues are injured. NO is a non-desired by-product of combustion. In the presence of water NO is oxidized to HNO.sub.3 and contributes in this form to about half of the acidification, the other half being caused by H.sub.2 SO.sub.4. A problem is that NOx can decompose ozone in the upper part of the atmosphere.
A great part of NO comes from the reaction between the oxygen and nitrogen of the air at a high temperature and is therefore independent of the composition of the fuel. How much NOx that is formed is besides dependant of during which conditions the combustion takes place. If the temperature reduction can take place very slowly this leads to an equilibrum at moderate high temperatures and a lower final concentration of NO.
The following methods can be used for keeping the formation of low NO
1. Combustion in two steps of a fuel rich mixture PA0 2. Low combustion temperature through
a) high excess of air PA1 b) strong cooling PA1 c) re-circulation of the combustion gases
In the chemical analysis of flames it has often been observed that the concentration of NO in the flame is much higher than thereafter. There is a process which decomposes NO. A probable reaction is: ##STR1## Thus the formation of N.sub.2 is supported by conditions which give high concentrations of CH.sub.3, viz. a hot, fuel rich flame.
Fuels containing nitrogen, for example in the form of heterocyclic hydrocarbons like pyridine from experience gives off more NO.
The content of N in different fuels (approximatively):
______________________________________ Crude oil 0.65% Asphalt 2.30% Heavy oils 1.40% Light oils 0.07% Carbon 1-2% ______________________________________
In SE-B-429.201 there is disclosed a liquid composition containing 1-10% by volume of hydrogen peroxide, at which the residual amount consists of water, aliphatic alcohol and lubricating oil and possibly a corrosion inhibitor, said liquid composition being supplied to the combustion air or the fuel-air mixture. At such low contents of hydrogen peroxide an insufficient amount of OH-radicals are formed for reaction both with the fuel and the CO formed. Besides there is no self-ingnition of the fuel obtained, at which the improvement of the combustion which is achieved as compared to the addition of only water is small.
In DE-A-2.362.082 there is described admixture of an oxidizing agent, for example hydrogen peroxide, in connection with combustion, however the hydrogen peroxide is decomposed to water and oxygen by means of a catalyst before it is supplied to the combustion air.